1. Field of the Invention
The present invention relates to a channel structure, a method for forming the channel structure, a radio base station, and a mobile station in a mobile communication system able to improve system capacity and communication quality.
2. Description of the Related Art
In specifications set by 3GPP (3rd Generation Partnership Project), which is one of the standards organizations on mobile and wireless communication systems for establishing standards of the wideband CDMA (W-CDMA: Wideband-Code Division Multiple Access), the so-called Downlink Shared Channel (DSCH), the shared channel used in downlink (a link from a base station to a mobile station) data communication, is defined as one of the channels shared by a plurality of users (mobile stations). Because DSCH is allocated to every mobile station per unit transmission (for example, for every frame) for use, it is expected to be used in high speed downlink packet transmission.
In addition, in 3GPP, studies are being made in standardizing HSDPA (High Speed Downlink Packet Access) that enables realization of high speed downlink packet transmission (about 8-10 Mbps). HSDPA is a transmission scheme for adaptively and fast changing modulation schemes (16 QAM, adaptive modulation coding scheme and so on) in response to variations of the propagation environment, or error correction coding rates. By adaptively controlling these modulation schemes, it will be possible to use the existing DSCH as a high-speed shared channel. In 3GPP, the high-speed shared channel used in HSDPA is defined as a “high speed downlink shared channel (HS-DSCH)”. Next, structure and functions of channels used in HSDPA are explained.
(Channel Structure of HSDPA)
1. Downlink Shared Channel (HS-DSCH)
2. Primary Common Pilot Channel (P-CPICH)
3. Secondary Common Pilot Channel (S-CPICH)
4. Associated Dedicated Physical Channel (A-DPCH)
(Explanation of Functions of the Above Channels)
HS-DSCH is a downlink communication channel shared by a plurality of mobile stations, and is allocated to each different user (a mobile station) at a short interval (for example, at every 2 ms). In W-CDMA systems, pilot channels for constantly transmitting signals from a base station providing services include common pilot channels (CPICH), and P-CPICH and S-CPICH belong to CPICH. P-CPICH and S-CPICH are reference channels used as reference symbols when a mobile station measures the downlink quality or performs coherent detections. In order to use P-CPICH as a phase reference signal for PCCPCH (Primary Common Control Physical Channel) for cell selection or annunciation information transmission in a mobile station, it is necessary to send signals to the entire sector of a radio base station. In contrast, P-CPICH is able to transmit signals in designated directions. A-DPCH includes a pair of channels for uplink and downlink set up between each mobile station and the base station for communications with HS-DSCH. In uplink direction, in addition to uplink user data, control data such as the downlink quality information and the TPC (Transmission Power Control) bit for downlink A-DPCH are also transmitted. On the other hand, in downlink direction, TPC commands for uplink A-DPCH, or information for call control are transmitted.
When HS-DSCH is used in a radio base station having a beam forming antenna able to switch the beam direction to each transmission destination, that is, each user, in HS-DSCH channel estimation (that means estimation of variations of phases and amplitudes of received signals using reference signals), the same pilots as that used in A-DPCH channel estimation is used. Accordingly, when the HS-DSCH is used in the radio base station having the beam forming function for forming the specific beam for each mobile station, one of the following two methods are used.
1). As shown in FIG. 13, radio base station 100 does not transmit S-CPICH, and the mobile station A201 uses the dedicated pilots arranged in A-DPCH for channel estimation during demodulation of HS-DSCH and A-DPCH, while the mobile stations B202 and C203 use the same for channel estimation during demodulation of A-DPCH.
2). As shown in FIG. 14, radio base station 100 transmits S-CPICH to each user (mobile stations A201, B202, C203), and when HS-DSCH and A-DPCH are demodulated, not only A-DPCH, but also S-CPICH are used for channel estimation. Accordingly, pilots the same as the dedicated channel associated with the shared channel are used in channel estimation of the shared channel.
Japanese Unexamined Patent Publication (Kokai) No. 11-88941 discloses a technique related to the pilot channel setting of communication channels. In this technique, it is proposed that a high-speed communication channel use the dedicated pilot channel associated with each communication channel, and a low speed communication channel use a common pilot channel shared by a plurality of communication channels.
However, in the above two methods for channel estimation, when the former one is used, there exists a probability the received power is not sufficient for HS-DSCH channel estimation, in other words, since A-DPCH is a relatively slow channel and transmitted by lower electric power, the power carried by the dedicated pilots may also be insufficient. In this case, the accuracy of channel estimation declines. The reception characteristics do not deteriorate too much for low power channels such as A-DPCH, even if the accuracy of channel estimation declines, but it does degrade noticeably for high transmission speed and high power channels such as HS-DSCH. Furthermore, when this method is used, it is impossible to measure the downlink quality by a mobile station.
In addition, when the latter one of the above two methods is used, system capacity may degrade because of increments of interference arising from transmission of S-CPICH to every user. In other words, since a large portion of the power transmitted from a radio base station is allocated to S-CPICH, the transmission power of other channels (including HS-DSCH) becomes low, and this leads to degradation of the system capacity. Furthermore, if more S-CPICHs are allocated, since more channelization codes have to be used, code resources may become insufficient, and consequently, degradation of the system capacity resources happens due to the insufficient codes.